Within the field of cellular communications, the increasing demand for capacity has resulted in increased use of more advanced antenna technologies. In particular, the use of so-called antenna arrays or array antennas has received increasing research activity. An antenna array is typically a group of similar antenna, elements, or antenna sections, that are arranged in various configurations with proper amplitude and phase relations in order to give certain desired radiation capabilities. The direction and shape of the antenna beam is determined by weighting each column signal with appropriate phase and amplitude factors. A receiving and transmitting antenna array comprises a number of receiving and transmitting antenna sections, and each transmitting or receiving path includes components e.g. feeder cables, beam formers, filters, radio units etc. that can distort the phase and amplitude of received and/or transmitted signals. In order to accurately shape and direct the antenna beams, these receiving and transmitting antenna arrays need to be accurately calibrated, such that any distortion of phase and amplitude, or time delay, of signals are corrected before transmission and after reception of the signals.
If several antennas with a same polarization are used, usually the antenna signals have to have a well-defined phase relationship. In order to ensure this, each radio path must be calibrated against the other paths with respect to phase. Different antenna branches can e.g. have slightly different feeder lengths, which affect the phase of the antenna branch. The internal analog filters might also be different. The differences can be temperature dependent and will change over time. This makes continuous calibration of the transmitter and in some cases receiver paths necessary. The characteristics will change slowly, so the calibration can be done with a low repetition rate.
Typically, the calibration procedure will calibrate a combination of a radio and the connected antenna (and the connecting feeder cable or network) and consists of several steps.                Measurement and calculation of relative transfer function between branches.        Calculation of compensation coefficients.        Applying coefficients and performing the compensation.        
One known manner in which to calibrate the phase of the antenna elements in an antenna array is to include a dedicated calibration coupler unit (CCU) which is built into the antenna, see FIG. 1. As illustrated in FIG. 1, a base band processing unit is connected to a radio unit via the CPRI interface. In the example four antennas (A0, . . . , A3) are connected to the radio, and the CCU is connected to each antenna. The CCU provides a calibration input/output for calibration measurements. When performing measurements on the downlink (DL), the CCU is used to provide a phase accreted connection to all antennas. The signals from all antennas are added and sent into a calibration receiver. The CCU itself is calibrated in advance. In addition, the radio e.g. transceiver units, may contain special hardware (HW) dedicated to support calibration measurements.
In other antennas, e.g. most four-antenna systems, no CCU is available. One possible solution would be to connect an external CCU unit to the antenna. Another possibility would be to include the CCU into a TMA. A further possibility would be to incorporate the CCU into the radio transceiver. However, in this case the feeders connecting the transceiver and the antennas would need to be matched and very phase stable. Since the feeder cables are not part of the calibration loop, the phase error would be large. As an example, for a 2 mm difference in feeder length, the phase error would typically be 10 degrees for a GHz system.
Consequently, there is a need for an improved method of phase calibrating the respective transmit and/or receive paths of antenna elements of an antenna array without necessitating a CCU in the system.